Process for the production of alkynes



Patented Feb. 27, 1951 UNITED STATES PATENTOFFICE PROCESS FOR THE PRODUCTION OF ALKYNES No Drawing. Application September 29, 1947; Se-

rial No. 776,852. In Great Britain October 31,

6 Claims.

This invention relates to an improvedprocess for producing alkynes by the dehydrohalogenation of halogen substituted alkanes and alkenes.

It is known to produce acetylene and the homologs thereof comprising the alkyne series of hydrocarbons by the elimination of hydrogen halide from halogenated hydrocarbons, as ethylene dibromide or mono-bromoethylene. However, the yield of alkyne obtained in this manner is small, frequently being but 20% to 30% of the maximum theoretical yield. This is particularly true when dealing with halogenated hydrocarbons containing four ormore carbon atoms and is largely due to the uncontrolled nature of the reaction whereby other products than the desired alkyne or alkynes are also produced. For example, when dehydrohalogenating ,2-dibromobutane with alcoholic potassium hydroxide, one obtains not only butyne-l and -2, but also 1- and 2-bromobutene-l, 1- and 2-ethoxybutene-l, and butadiene-1,2.

It is therefore an object of the present invention to provide an improved process for obtaining alkynes from halogenated hydrocarbons. A more particular object is to provide a process of this nature whereby dehydrohalogenation of the starting materials proceeds in a controlled man" nor, with resultant high yield of alkyne and reduction in Joy-product formation.

The process of the invention whereby the foregoing and other objects are attained is based on the discovery that greatly improved yields 1 of alkynes are obtained when the dehydrohalogenation of the halogenated hydrocarbon starting materials proceeds either in the absence ofwater, or with the prompt removal of any water which may be formed during the reaction.

When the reaction is to be conducted without water formation, an alkali metal alcoholate is employed as the dehydrohalogenating medium, particularly good results beingobtained when the alcoholate comprises an anhydrous, alkali metal derivative of a high boiling hydroxylic material, as, for example, the ethyl or methyl others of ethylene glycol, diacetone glycol, and diacetone glycol monomethyl ether.

Alternatively, it has been discovered that good results can also be obtained when thedehydrohalogenation is accomplished through' use of an alcoholic solution of alkali metal hydroxide provided that any water formed during the reaction is promptly and continuously removed. This removal may be effected in any one or more of several different ways. A preferred method is to add to the solution an organic liquid such as The water may be removed bybubbling a dry gas through the reacting solution, In some cases the alcohol solvent medium itself forms an azeotrope of satisfactory boiling point with the water formed. In still another method the halo-reactant itself, particularly the haloalkene, may act to entrain the water through formation of an azeotropic mixture, this method being of the greatest practical value when the reaction is carried out in the presence of a high-boiling hydroxylic solvent.

Insofar as the mechanism of reaction is concerned whereby either dihalogenated alkaries or monohalogenated 'alkenes are dehydro-halogenated to produce the corresponding alkynes, process of the present invention does not differ from those of the prior art. The dihalogenated alkanes are those wherein the halogen atoms, which are preferably chlorine or bromine, but may befluorine or iodine, are attached to the same or adjacent carbon atoms, whereas in the halogenatedalkenes the halogen atom is attached to oneorthe other of the carbon atoms linked bythe doublebondl For example, the alkynes appearing in the left hand column below may be obtained by dehydrohalogenating any of thehalogenatedhydrocarbons set against them in the right hand column, and in most cases other'starting materials than those listed could have been used aswell:

l-bromohcptone-l; 2-chloroheptcnc-l.

While the process of the invention is broadly applicable to thepreparation of all members of the alkyne seriesQit is preferably used only in the preparation of those alkynes having at least four carbon atoms. The 'dehydrohalogenation process here described is practiced in the solution phase, and those alkynes and halogenated alkanes and alkenes having less than four carbon atoms have such a'low-boiling point that it is dimcult to retain them in the liquid phase except at low temperatures. However, the reaction by which'halogenated alkanes and alkenes are convertedintoalkynes through removal of one or two molecules of hydrogen halide, respectively,

is an endothermic one. It is normally conducted at temperatures above '?5 C., and for periods which may range from several minutes to two or three hours.

However the dehydrohalogen'ation of the prespound is used, in the one case to dissolve the alcoholate and the halogenated alkane or alkene starting material, and in the other case to dissolve the alkali metal hydroxide and the said starting materials. By hydroxylic compounds it is meant to include alcohols and other organic compounds related thereto which contain the (OH) group, and of the compounds coming within this class it is preferred to use those having a boiling'point in excess of 100 C. While a wide variety of hydroxylic compounds, including amyl, isoamyl, hexyl and isohexyl alcohols, as well as such polyhydric alcohols as ethylene glycol and propylene glycol, for example, are satisfactory for use in the process of the present invention, it is preferred to employ an ether derivative of a polyhydric alcohol as, for example, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, glyceryl monomethyl ether, glyceryl monoethyl ether, glyceryl dimethyl ether, glyceryl diethyl ether, diacetone glycol monomethyl ether and beta-methyl, glyceryl monomethyl ether. All the compounds mentioned in the paragraph fall within the term hydroxylic compounds and have boiling points in excess of 100 C.; most of them boil above 150 C.

The anhydrous'alkali metal alcoholates employed to dehydrohalogenate the halogen substituted alkane or alkene starting materials may be prepared by dissolving the desired quantity of alkali metal hydroxide in an excess of the hydroxylic compound with heating. It is preferred that the hydroxylic compound employed be a high-boiling ether derivative of a polyhydroxylic .material, as the monoalkyl ether derivatives of ethylene glycol, for example, and of the various alkali metal hydroxides, those of sodium or potassium are preferred, though the more expensive cesium, lithium, and rubidium hydroxides can be used. The solution is then cooled and any water present is removed. This can be most conveniently and thoroughly accomplished by adding a liquid organic compound such as benzene, toluene or xylol, for example, which is capable of forming a low boiling azeotrope with water and then subjecting the mixture to slow distillation 7 until all water present has been azeotropical1y distilled off leaving the alkali metal alcoholate dissolved in the excess of hydroxylic compound. Any residue of benzene or other organic liquid added may be removed by distillation or left in the solution. r

In dehydrohalogenating partially halogenated hydrocarbons to produce alkynes through use of anhydrous alcoholate solutions of the type described above, suitable alkane or alkene starting materials are first dissolved in the alcoholate solution, care being taken to make sure that the said starting materials themselves contain no water. If there is doubt on this score, the materials may be dried by subjecting them to a preliminary distillation step, or by other known means. With either type of starting material i. e., dihalogenated alkane or monohalogenated alkene, it is essential for the highest yields of alkyne product that the reactants be combined in such proportions that the amount of alkali metal present asa component of the alcoholate be at least equivalent to the amount of halogen present as a component of the starting materials, and preferably there should be present an excess of the alkali metal, 1. e., an excessof the alco- This excess may range anywhere from 4 about 5 to as much as 100%, or even more. For example, when dehydrohalogenating 1,2-dibromohexane to a hexyne through use of the potassium alcoholate of ethylene glycol monoethyl ether, there should be used at least two moles of the alcoholate for each mole of the alkane, and were l-bromohexene-l to be the starting material, there should be used at least one mole of the alcoholate for each mole of the alkene. It is also possible to employ the anhydrous alcoholate solution to eiTect partial dehydrohalogenation of,

the dihalogen alkanes and thereby form the monohalogen substituted alkene; when the 'latter product is that desired, the quantity of alkali metal present should be equivalent to but onehalf of the halogen present in the dihalogen alkane, and no excess of alkali metal should be used. l-bromohexehe-l, for example, may be formed in this manner from dibromohexane through the use of one mole of the dibromohexane and one mole of the alcoholate. However, it is not necessary to the formation of the monohalogen substituted alkene that the reaction be carried on in the absence of water, a fact which will also be discussed in a succeeding portion of this description.

Following the dissolution of the halogen substituted hydrocarbon starting materials the solution is heated to bring about the dehydrohalogenation desired. This may be accomplished by maintaining the solution at or above the temperature at which the alkyne boils and recovering the latter as it is distilled oif; alternatively, the alkyne may be separated by solvent extraction or other physical means, though the distillation method of separation is preferred.

For best results, the heating stepby which the alkyne is formed should be carried on at temperatures above 75 C., and preferably above 100 C. The solution should be maintained at such elevated temperatures until dehydrohalogenation is complete, the alkyne product being dis- .tilled oif substantially as fast as it is formed.

The dehydrohalogenation reaction for the production of alkynes which i described in the preceding paragraphs proceeds without the formation of water and therefore no provision need be made to eliminate this component, a sample reaction being.

OHa-CHrCHz'CHCl-CHaCI ZCzHsOCHrCHgOK 1,2-dichloropentane potassium alcoholate of ethylene glycol monoethyl ether ethylene glycol monoethyl ether This reaction is a Very efficient one, and totalv alkyne yields as high as to of the maxi-- mum theoretical yield have been obtained, the. yield of the alkyne-1 product being as high as '75. to 80%.

When formation of the alkyne is not effected through use of an anhydrous alcoholate solution, the alkyne may be produced by dissolving the halogenated alkane or alkene starting material in a solution of alkali metal hydroxide in a hydroxylic solvent, and thereafter heating the solution with simultaneous elimination of water. 'Representative solutions of the type here under consideration are those formed by dissolving either 1,2-dibromohexane or 2-monobromohexene-l and potassium hydroxide or sodium hydroxide in a solvent such a ethylene glycol monomethyl' ethyl or ethylene glycol monoethyl ether. In preparpentyne-l ing these solutions, careshouldbe taken to make 5 sure ha the kali m tal h dro ide s ad e that a d he s lu ion eqoled. before he a o en substitu e s tin mate ial s t o u ed- A am e re tion. o he pe he ncounte e s,

in presence of ethylene glycol monomethyl other w en? GHa- Hz-CHWET +.ZKQ 2 32.

' h i e- With e t ree Q sta in mate ia a. diha en d kane r onqrha en ten al en th a t t h d b to, .bined in u proportions that the amount of alkali metal hydroxide present is at least equivalent to the amount of halogen present in the hydrocarbon starting ma terial, and it is preferred that there be 'an excess of alkali metal hydroxide present over this equiv,- alent amount required to form alkali metal halide salts with the halogen constituent of the said starting material. In using this method of dehy drohalogenation, the same conditions of reaction should prevail as were noted above for use with the anhydrous alcoholate solutions, 1. e. a reaction temperature above 75 C., and preferably above 100 0,, with the elevated temperatures being maintained until the reaction is substantially complete. Once the reaction is begun, it is important to an efficient production of the alkyne that any water present be removed and kept removed as fast as it is formed. In some cases this may. be accomplished by bubbling a dry inert gas through the liquid, particularly when the solution is heated at temperatures in excess of 100 C. Another method of removing the water which may CH CHa-CH -OHOl-CHaQl -l- 2KQH 1,2-dich1oropentan be practiced inan efficient manner when the reaction is carried out at temperatures above 100 C. is to utilize a portion ofthehalogen substituted reactant present, and particularly the monohalogen alkene, which is either added as a starting material or formed asan intermediate product from the dihalogen alkane, to form a water azeotrope. Still another. method of removing the water is to add to the solution an organic liquid such as, for example, benzene, toluene. or. amyl alcohol, the added liquid being one which is capable of forming an azeotrope boiling below 100 C. with any water present. Forv example, toluene, which boils at 110.80, forms a. water azeotrope boiling at 84.1. C The various amylalcohols have even higher. boiling points, while still forming water azeotropes of a relatively low. boiling point. In many cases it provesconvenient to add a succession of amounts of the non-aqueous azeotrope forming liquid. When the water present is eliminated inthemannerdescribed above, it does not remain in the solution long enough to interfere with the reaction, and as a result, greatly improved yields of the desired alkyne produce are obtained. While the process of this reaction is not so, efiicient as that wherein anhydrous alcoholate olutions are employed asthe dehydrohalogenation medium, nevertheless total alkyne yields in excess of 75% of the maximum theoretical yield have been obtained.

In a modification of the foregoing feature of the invention, dihalogen alkanes such asare. obtainable by halogenating alkene-l products containing at least. 4 carbon. atoms in the molecule, are first treated with an alkali metal hydroxide on a mole-for mole basis. in the. presence... of a suitable hydroxylic solvent medium; and preferably, one of the higher-boiling. type. as. glycol monoethyl ether or glycol monomethyl ether, in order to efiect dehydrohalogenation to the monohal eenzal-ken s w ic a e t her d rom the re n tion m xturei or example, y olv nt extract on o by the convent ona d st llati n p c d re or by tillat on with team- W hav o n t a t is r ac ion s Qne whic may be car i d ou with u i i t Qn of: w ter but it a o b un t at his e cti n p oceeds n the mo efiicie t ma ner when he he -h ili rdroxylic mate ia s. Such as he gl co mon alk others are utilized as the solvent medium The monohalos n k nes thus o aine i d si e af r sepa tion b ract o l st at on a t en tre t with n al al e a hydr d in the presence of a glycol monoalkyl ether or other ehebo g hyd xy i compoun in w ic the monohalogen alkenes are sufficiently soluble, and the resulting mixture is thendistilled under conditions whereby alkyne l products are distilled- Q su stant a y as form d, wh a y er p ent or produced is continuously removed by entrainment or azeotropic distillation with higher.- boiling components of the mixture. In many cases, the monohalogen alkene component itself serves to form an azeotropic mixture, and the reaction may be carried out without the :addition of other materials, a portion of; the said monohalogen alkene leaving the solution in the form of an azeotrope while the balance is converted into the desired alkyne product.

The following. examples illustrate the manner in which the present invention find application:

Example I t a de d o e araheh ner om m ture of l.,chlorohexene-1 and Z-chlorohexene-l. An anhydrous alcoholate solution was prepared by dissolving 21 grams of potassium hydroxide in 108 cc. of ethylene glycol monoethyl ether, the solution being maintained at a temperature of about 100 C. during the course of this addition. The mixture was then cooled to about 50 C.., and cc. of benzene were then added. The resulting mixture was then subjected to slow distillation whereby all water present was azeotropically distilled off leaving the anhydrous potassium derive ative of ethylene glycol monoethyl ether dissolved in the excess, of ethylene glycol monoethyl ether.

' To the alcoholate solution obtained in themanner described above. were added 28 grams of a water-free mixture'of l and 2 .-chlorohexene-1, obtained by heating a mixture of parts of l,2..-dichlorohexene. (obtainable by chlorinating hexeneel) and 25 parts of potassium hydroxide dissolved in 150 .parts of glycol monoethyl ether on a water bath for 2. hours and then steamdistilling the resulting mixture to recover the 1- chlorohexene-l and 2-chlorohexene-1 products. The alcoholate chlorohexene solution obtained in. this manner. was then gradually heated until it reached 'a temperature of Q, and it was maintained at this temperature for a period of 1.5 hours. During this time. the hexynes formed were distilledoff and. recovered by condensation. A quantity of approximately 15'grams of heXyne lboiling. tit-71. C. was obtained in this manner, representing ayield of; about 78%,011 the maximum theoretical yield. However, other hexynes thanhexyne-l were also recovered in the amount of approximately 3. grarns sothat thetotal hexyne weaveseaithe-max m m; he e i l i l Eenmhleh'.

A mixture of heptynes consisting predomach eve 'i'nantly of heptyne-l' may be prepared from 1,2-

"tially all of its water in the form of a water toluene azeotrope as the temperaturereaches-approximately 84 to 85 C., and with continued heating above this temperature small amounts of the azeotrope will be continuously recovered as the -l,2 -dibromoheptane is converted into the form of the various heptynes. By the time the solution has been heated forone or more hours at a temperature between 130' and 140 C., the desired reaction will be substantially complete and approxi- -mately grams of mixed heptynes will have been recovered as a distillate, of which approximately 12 grams will be heptyne-l. This represents a heptyne-l yield of 65%.

' We claim as our invention:

l. The method of forming an alkyne from a halogenated hydrocarbon, said method comprising forming an anhydrous alcoholate of sodium hydroxide and ethylene glycol monoethyl ether, dissolving said alcoholate in an excess of said ethylene glycol monoethyl ether, dissolving in the resulting solution a halogenated hydrocarbon selected from the group consisting of alkanes having two halogen atoms attached to a single carbon atom, alkanes having a halogen atom attached toeach of twoadjacent carbon atoms, and alkenes having a halogen atom attached to an unsaturated carbon atom, the content of sodium in said alcoholate being at least equivalent to the content of halogen in said hydrocarbon, the solution so formed being anhydrous in nature, and heating saidl anhydrous solution at a temperature above 75 C. to eiiect dehydrohalogenation of said halogenated hydrocarbons to form said .alkyne.

2. The method of forming an alkyne from a halogenated hydrocarbon said method comprising forming an anhydrous alcoholate of potassium hydroxide and ethylene glycol monoethyl ether, dissolving said alcoholate. in an excess of said ethylene glycol monoethyl ether, dissolving inv the resulting solution a halogenated hydrocarbon selected from the group consisting of alkanes having two halogen atoms attached to a single carbon atom, alkanes having a" halogen atom attached to each of two adjacent carbon atoms, and alkenes having a halogen atom attached to an unsaturated carbon atom, the content of potassium in said alcoholate being at least equivalent to the content of halogen in said hydrocarbon, the solution so formed being anhydrous in nature, and

tached to each of two adjacent carbon atoms,

and alkenes havinga halogen atom attached to 8 metal in said alcoholate being at least equivalent to the content of halogen in said hydrocarbon, the solution so formed being anhydrous in nature, and heating said anhydrous solution at a temperature above C. to efiect dehydrohalogenation of said halogenated hydrocarbons to form said 'alkyne. I

4. The method of forming an alkyne from a halogenated hydrocarbon, said method comprising formingjan' anhydrous alcoholate of an alkali metal and an ether derivative of a polyhydric alcohol having a boiling point in excess of C.,

dissolving said alcoholate in an excess of said alcohol, dissolving in the resulting solution a halogenated hydrocarbon selected from the group consisting of alkanes having two halogen atoms attached to a single carbon atom, alkanes having a halogen atom attached to each of two adjacent carbon atoms, and alkenes having a halogen atom attached to an unsaturated carbon atom, the solution so formed being anhydrous in nature, and heating said anhydrous solution at a temperature above 75 C. to efiect dehydrohalogenation of said halogenated hydrocarbons to form said alkyne.

5. The method of forming an alkyne from a halogenated hydrocarbon said method comprising forming an anhydrous alcoholate of an alkali metal and a hydroxylic liquid having a boiling point in excess of 100 C., dissolving said alcoholate in an excess or" said liquid, dissolving in theresulting solution a halogenated hydrocarbon selected from the group'consisting of alkanes' having two halogen atoms attached to a single carbon atom, alkanes having a halogen atom attached to each of two adjacent carbon atoms, and alkenes having a halogen atom attached to an unsaturated carbon atom, the content'of alkali metal in said solution being at least equivalent to the.

content of halogen attached to said hydrocarbon, the solution so formed being anhydrous in nature, and heating said anhydrous solution at a temperature above 75 C. to effect dehydrohalogenation of said halogenated hydrocarbons to form the alkyne.

6. The method of forming alkynes comprising 7 forming an anhydrous solution of a monohaloalkene having a halogen atom attached to an unsaturated carbon atom thereof in an anhydrous solution of an alkali metal alcoholate and heating the resulting monohalo-alkene-containing solution.

RAYMOND SPENCER AIRS.

PHILIP JAMES GARNER.

REFERENCES CITED The following references are of record in the file of thispatent:

' UNITED STATES PATENTS OTHER REFERENCES an unsaturatedcarbon atom, the content of alkali Adkins et 'al.: Organic Synthesis, vol. 26, pages 21-3 (1946). 

5. THE METHOD OF FORMING AN ALKYNE FROM A HALOGENATED HYDROCARBON SAID METHOD COMPRISING FORMING AN ANHYDROUS ALCOHOLATE OF AN ALKALI METAL AND A HYDROXYLIC LIQUID HAVING A BOILING POINT IN EXCESS OF 100* C., DISSOLVING SAID ALCOHOLATE IN AN EXCESS OF SAID LIQUID, DISSOLVING IN THE RESULTING SOLUTION A HALOGENATED HYDROCARBON SELECTED FROM THE GROUP CONSISTING OF ALKANES HAVING TWO HALOGEN ATOMS ATTACHED TO A SINGLE CARBON ATOM, ALKANES HAVING A HALOGEN ATOM ATTACHED TO EACH OF TWO ADJACENT CARBON ATOMS, AND ALKENES HAVING A HALOGEN ATOM ATTACHED TO AN UNSATURATED CARBON ATOM, THE CONTENT OF ALKALI METAL IN SAID SOLUTION BEING AT LEAST EQUIVALENT TO THE CONTENT OF HALOGEN ATTACHED TO SAID HYDROCARBON, THE SOLUTION SO FORMED BEING ANHYDROUS IN NATURE, AND HEATING SAID ANHYDROUS SOLUTION AT A TEMPERATURE ABOVE 75* C. TO EFFECT DEHYDROHALOGENATION OF SAID HALOGENATED HYDROCARBONS TO FORM THE ALKYNE.
 6. THE METHOD OF FORMING ALKYNES COMPRISING FORMING AN ANHYDROUS SOLUTION OF A MONOHALOALKENE HAVING A HALOGEN ATOM ATTACHED TO AN UNSATURATED CARBON ATOM THEREOF IN AN ANHYDROUS SOLUTION OF AN ALKALI METAL CLCOHOLATE AND HEATING THE RESULTING MONOHAL-ALKENE-CONTAINING SOLUTION. 